That's not correct by the way. I thought so too for a while, but the mmcblk0/1 corresponds to the order you insert the cards. So if you first insert a card in the right slot and afterwards in the left slot, the naming will be reversed!(the left card is /dev/mmcblk0, the right card is /dev/mmcblk1)
It does tell you the size of the card though (which will probably a be bit less than what's written on the card, e.g. 59,58 GB instead of 64 GB, that's normal!) This will help you to find the right one.
But don't worry: it is still fool-proof since (as WizardStan said) you can not format the card you started gparted from. And the SD slots are all you have access to from gparted (plus USB sticks maybe), so you can't mess up anything important.
Just stick to the rest of WizardStan's description and you'll be fine. I was pretty intimidated when I tried this for the first time, but it actually turned out to be very easy. You'll be able to format anything to whatever you want and even make partitions once you know how gparted works. so it really is worth trying.
About the SDHC/SDXC stuff ... yeah, well all I wanted to say is that some very very (very?) old card readers have problems with "modern" SD cards, but if a 32GB card works this is probably not an issue.